drm: Move drm_vm_open_locked into drm_internal.h

Leftover from my previous header cleanup.

This depends upon the patch to rework exynos mmap support, otherwise
it'll break exynos.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>